Modern gas turbine aircraft engines have a variety of accessories mounted on their external cases. Examples of external accessories include control systems for regulating the engine's operation, heat exchangers for cooling oil or heating fuel, and pumps for circulating fluids.
Various arrangements of hardware are used to mount an accessory on an engine. For example, an accessory may include brackets which are fastened to engine case flanges by bolts extending through holes in both the brackets and the flanges. The accessory is mounted on the engine by manually positioning the accessory so that the bracket holes are in close proximity to and in alignment with the corresponding case flange holes. While the accessory is held in position, bolts are passed through the bracket and case flange holes. The bolts are secured by nuts to connect the accessory to the engine.
Alternative mounting arrangements in which a fastening means, such as the nuts and bolts of the foregoing example, is engaged while the accessory is manually held in position are also possible. Such mounting arrangements may be effective for compact, light weight accessories. Many accessories, however, are unwieldy due to their bulk and weight. A mechanic attempting to mount such an accessory may need both hands to position the accessory, leaving neither hand free to fasten the accessory to the engine. An attempt to maintain the accessory in proper position with one hand may result in the accessory shifting out of position or possibly being dropped and damaged. The difficulties of mounting a heavy or bulky accessory are further exacerbated if the accessory is to be mounted in a difficult to reach location on the engine case or if the fastening means is difficult to reach once the accessory is properly positioned.
It is also desirable to minimize the quantity of loose parts associated with any accessory mounting arrangement. A loose part, such as a bolt or nut, can easily be lost or misplaced. Far from being a mere inconvenience, loose parts can be ingested into turbine engines causing considerable engine damage.
It may also be desirable to protect such a case mounted accessory from the potentially detrimental effects of case deflection or distortion caused by structural loads acting on the case or by case temperature nonuniformities. Externally mounted accessories are often connected to the engine case at two or more locations along the length of the engine. During engine operation, the internal temperature of the engine is different at different positions along the engine's length so that the case expands and contracts by unequal amounts at each mounting location. If an accessory is connected to the engine by a hard mount (that is, a connection which is essentially inflexible and immobile such that case deflections are transmitted to the accessory) the differences in the amount of radial expansion or contraction of the case at each of the mounting locations bends the accessory, the mounting hardware or both and imposes undesirable stress on the mounting hardware or on the accessory itself. Accordingly, it is known to use a floating mount, rather than a hard mount, at one or more of the mounting locations. A floating mount is a connection having some flexibility or mobility so that the accessory is protected from case deflections and distortions.
In view of the difficulty of mounting and dismounting an external accessory, the undesirability of loose parts, and the benefits of protecting an accessory from engine case deflection and distortions, a mount arrangement which simplifies the mounting and dismounting of the accessory, has no loose parts and offers protection from case deflections and distortions is sought.